1. Field of the Invention
The present invention relates to a fixing device, and more particularly to a fixing device for use in, for example, an image forming apparatus using an electrophotographic system.
2. Description of the Related Art
In an image forming apparatus, such as a copying machine or a printer, using an electrophotographic system, a photoreceptor (photosensitive drum) formed into, for example, a drum shape is uniformly charged, and the photosensitive drum is exposed with light controlled on the basis of image information to form an electrostatic latent image on the photosensitive drum. This electrostatic latent image is changed to a visible image (toner image) by toner, this toner image is transferred onto a recording sheet, and this is fixed by a fixing device to form an image.
A fixing device used for such an image forming apparatus includes, as shown in FIG. 6, a fixing roll 110 which has a cylindrical core metal 111 provided with a heat source 114 inside and is formed by laminating a heat resistant elastic layer 112 on the core metal 111 and by laminating a release layer 113 on the outer peripheral surface thereof, and a pressure roll 120 which is disposed to come in press contact with the fixing roll 110 and is formed by laminating a heat resistant elastic layer 122 on a core metal 121 and by laminating a release layer 123 of a heat resistant resin coating or a heat resistant rubber coating on the outer peripheral surface thereof. A recording sheet bearing a non-fixed toner image thereon is made to pass through between the fixing roll 110 and the pressure roll 120, and the non-fixed toner image is heated and pressed, so that a toner image is fixed on the recording sheet. The fixing device as stated above is called a roll nip system and is generally widely used.
In a case where an attempt is made to speed up the roll nip system fixing device, in order to supply a sufficient amount of heat to a toner and a recording sheet, it becomes necessary to widen a nip width in proportion to a fixing speed. As a method of widening the nip width, there is a method of increasing a load between a fixing roll and a pressure roll, a method of thickening the thickness of an elastic body, or a method of increasing a roll diameter.
However, in the method of increasing the load and the method of thickening the thickness of the elastic body, since the shape of the nip width becomes irregular along the roll axis because of the deflection of the roll, there arises such a problem in image quality that uneven fixation or a paper wrinkle occurs. In the method of increasing the roll diameter, there is a problem that the device becomes large, and a time (warm-up time) taken to heat the roll from room temperature to fixable temperature becomes long.
In order to solve these problems and to realize a fixing device capable of speeding up an image forming apparatus, the present applicant has proposed a technique relating to a fixing device which includes a fixing roll having a surface coated with an elastic body and an endless belt stretched by plural support rolls, and in which the endless belt is wound around the fixing roll over a specified angle area to form a nip area between the endless belt and the fixing roll, and a pressure is locally applied to an outlet of the nip area, the pressure being higher than that of the other portion of the nip area, to produce deformation in the elastic body of the fixing roll (refer to Japanese Patent No. 3,084,692, pages 5 to 8).
Further, the present applicant has proposed a technique relating to a fixing device which includes a rotatable fixing roll having an elastically deformed surface, an endless belt capable of running while being in contact with the fixing roll, and a pressure pad disposed in a non-rotation state at the inside of the endless belt, and in which the endless belt is brought into press contact with the fixing roll by the pressure pad so that a contact surface with the fixing roll is formed, a belt nip for allowing a sheet to pass through is provided between the endless belt and the fixing roll, and a portion of the surface of the fixing roll at the outlet side of the sheet is locally elastically deformed (Japanese Patent No. 3,298,354, pages 4 to 7).
In the technique disclosed in Japanese Patent No. 3,084,692, the belt nip is formed by the contact of the endless belt stretched by the plural rolls. In the technique disclosed in Japanese Patent No. 3,298,354, the endless belt is brought into press contact with the fixing roll by using the pressure pad so that the belt nip is formed. By adopting the structure as stated above, the width of the belt nip formed by the fixing roll and the endless belt can be easily made larger than the width of the roll nip between the conventional fixing roll and the pressure roll. Thus, it becomes possible to deal with speeding-up, and it is also easy to miniaturize the device.
Especially, since the heat capacity of the endless belt brought into press contact with the fixing roll is small, heat transmitted from the fixing roll is hard to dissipate. Thus, even if the rotation of the fixing roll is started, the amount of heat removed from the fixing roll to the endless belt side is relatively small, and the efficiency of using heat for the melting of toner becomes high, and therefore, there is also a merit that the fixation of the toner can be improved.